Color management is becoming more and more important since nowadays not only professional designers and illustrators but also average consumers wish to reproduce color as faithfully as possible by means of their electronic color output devices. This development is, among other things, the consequence of a rapidly increasing number of users taking pictures with digital cameras and printing the pictures, e.g. with their desktop color inkjet printers. One crucial point in terms of color management is that each color recording or reproducing device has its own device-dependent color space by reference to which it records or reproduces colors. For example, two digital cameras of different manufacturers taking the same picture under the same lighting conditions will store different RGB-values in their memory due to the differences between their photo-sensors, lens-systems and color processing firmware. Therefore, in order to be able to compare RGB-values of different color input devices, the colors are integrated into one device independent color space, which is usually the CIE (Commission Internationale de l'Eclairage)-LAB-color space or the CIE-XYZ color space. Another example would be two color inkjet printers of two different manufacturers operating in a CMYK-color space having the primaries cyan (C), magenta (M), yellow (Y) and black (K). If the same CMYK-values are sent to the different color printers, different colors will appear on the print medium with regard to the LAB-color space. The LAB-values of color patches printed on a print medium can be measured e.g. with a spectrophotometer which yields the LAB-color values of the color patches. A patch should be understood as a region of the media which is uniformly filled with a color.
If LAB-values of some color patches are measured, the printer-related CMYK-values of which are known, LAB-values can be determined for all possible CMYK-values by means of a mathematical transformation. To this end, color values are transformed from the device-dependent color values (e.g. RGB-color space of a scanner or digital camera) into the device-independent color values of the LAB-color space. This transformation may be encapsulated by means of an ICC profile which represents a mapping from the device-dependent color space of a color device into the LAB-color space and vice versa. The transformation can be later performed by a Color Management Module, or CMM. To this end, a “neutral observer” is applied which is able to measure a color in the LAB-color space. This neutral observer is typically a calorimeter which uses filters that mimic the neutral observer's response or a spectrophotometer which measures the wavelengths of the reflected light of color patches and calculates the corresponding LAB-color values. The reflected light is typically produced by a controlled light source of known characteristics. Typically, a spectrophotometer is an external device, which measures the device-independent LAB-color values of a set of color patches. The spectrophotometer typically is a handheld device. Typically, a spectrophotometer would measures spectra, Lab (or an other calorimetric representation) which may be calculated by means of an evaluation of the reflectance spectrum in view of the spectrum of a given illuminant.
A set of color patches printed by a color output device on the basis of known device-dependent color values is also referred to as a “target”. If the color patches have been produced, e.g. by an inkjet printer having its own CMYK-color space, whereby a point in the CMYK-color space represents a corresponding mixture of the four different inks cyan, magenta, yellow and black, then the CMYK-color points associated with the color patches can be assigned to the measured LAB- or XYZ-values of the patches to obtain a profile. In this context, the color values provided to the color output device are also referred to as “stimulus” and the color patches printed are referred to as “response” of the color output device to the color values. Thereby, a mapping is defined which maps the device-dependent color values (e.g. CMYK values) to the LAB- or XYZ-values of the color patches measured by a spectrophotometer. This mapping is often represented in the form of a matrix or a lookup-table (LUT), whereby a lookup-table representing a mapping from a device-dependent color space into a device-independent color space is usually referred to as “AtoB”-mapping, and a mapping from the device-independent color space into the device-dependent color space is usually referred to as “BtoA”-mapping. Often, a target has less color patches than a lookup-table has entries. Then, an interpolation of the measured color values is performed to populate the whole set of entries in the LUT.
Generally, the question arises how to measure color values of color patches. To this end, printing devices have been introduced which integrate an embedded color measurement device.
In U.S. Pat. No. 6,809,855 B2 an “improved and lower cost color spectrophotometer” is described, which is integrated in a color printer for on-line continuous color correction purposes.
US 2002/0080373 A1 describes a proofing printer with an embedded color measurement device for emulating a high volume output device. To this end, a target (“test image”) printed by the proofing printer is measured first to calibrate the proofing printer (“color calibration adjustments”), and then a target printed by the high volume output device is measured to modify the proofing printer's calibration so that it emulates the high volume output device (“color management adjustments”).
US 2005/0018219 A1 describes a digital printer with a built-in color measurement device in the form of a calorimeter or spectral photometer.
GB 2 409 122 A describes a printer, whereby a spectrophotometer is located within the printer.
The drawings and the description of the drawings are of embodiments of the invention and not of the invention itself.